Radio over fiber (RoF) networks for radio waves in the millimeter-wave frequency band are expected as a future infrastructure technology because an effective increase in the coverage of wireless access networks that use the radio waves is possible. Additionally, wavelength division multiplexing (WDM) technology is very attractive since in millimeter-wave frequency band RoF networks, the device configuration of a remote access unit (RAU) having an antenna disposed therein is simplified to minimize cost and power consumption and a remote node (RN) having a function of multiplexing and demultiplexing WDM signals is capable of widely distributing RoF signals to a large number of RAUs. Accordingly, millimeter-wave frequency band RoF networks that use WDM technology are low in maintenance cost as well as excellent in cost effectiveness, and can also provide low power consumption structures.
With the great increase in the demand for wireless access and its communication speed, the way in which mobile traffic is accommodated in cooperation with an optical access network (fixed network) has given rise to an important issue. From a physical point of view, one solution to this issue is the use of RoF technology. This technology is also receiving attention in the fronthaul standardization of wireless access networks. Nowadays, for example, this technology is drawing much attention also in the standardization of NG-PON2 (Next Generation Passive Optical Network 2), which is under study, in such a manner that a controversy over the accommodation of mobile traffic using the PtPWDM (Point To Point Wavelength Division Multiplex) scheme exists. In general, the fusion of WDM (Wavelength Division Multiplex) technology and this RoF technology is considered to become essential in the near future. Further, also in various wireless systems other than mobile ones (for example, broadcasting, standard radio waves, radar, wireless sensors, etc.), it is unavoidable that the fusion of WDM technology and RoF technology become a key constituent technology for the collection and distribution of wireless signals at a large number of points.
This requirement can be satisfied by realizing, for example, traffic control for a distributed antenna system (DAS) by using a conventional electrical cross-connect switch (XC-SW) disposed in a central station (CS). However, such electrical dynamic channel allocation (DCA) makes it difficult to completely avoid electromagnetic interference (EMI) between wireless signals within the CS-dedicated domain. Thus, dynamic optical channel allocation in which DCA is performed in the optical domain has become a solution for electromagnetic interference.
As a conventional dynamic optical channel allocation device, there is known, for example, a delivery-and-coupling optical switch board (DCSW: Delivery and Coupling Switch) illustrated in FIG. 1. The DCSW is configured to switch light paths by using thermo-optic switches (TO-SWs) and to couple beams of light by using optical couplers so as to perform optical signal distribution and combining in units of light paths but is not configured to perform the distribution and combining on channels having different wavelengths in the wavelength division multiplex transmission scheme. That is, the DCSW is configured to, when an optical signal including multiple optical channel components based on the wavelength division multiplex transmission scheme is input from each input port, dynamically allocate each input optical signal to any output destination by using a TO-SW, multiplex, by an optical coupler provided for each output port, respective optical signals output from the corresponding TO-SWs by using power combining, and output the resulting signal. In this conventional scheme, however, the switching of light paths is performed by repeating two splits, and there is a drawback with regard to the number of channels n in which as the number of channels increases, the splitting loss increases, which results in an increase in insertion loss. If the splitting loss affects system performance, it is inevitable to perform power supply with optical amplification or the like to compensate for the loss. In addition, since the characteristics of the optical coupler used do not include wavelength selectivity, there is no function for collecting multiple optical frequency channel components allocated as desired by using the wavelength division multiplex transmission scheme and for wavelength division multiplexing all the channel components into a single light path.
As a conventional scheme to overcome the foregoing situation, the use of an arrayed waveguide grating (AWG) instead of an optical coupler has also been proposed (PTL 1). However, since channel switching is performed for each wavelength, it is difficult to handle RoF signals including multiple wavelength components, signals with optical-frequency-interleaved channels, and the like.
The present invention also adopts a spectroscopic means such as the AWG described above, with the AWG being known to have the following input/output relationship (FIG. 5 in PTL 2). That is, when, as illustrated in FIG. 2(a), beams of light having wavelengths ν1 to νn are input to the uppermost one of input ports in an array, with regard to the corresponding output port, when input, given that ν1 to νn are sequentially output to the respective output ports, as illustrated in FIG. 2(b), for example, in response to beams ν1 to νn of light being input to the (n−1)-th port, ν1 and ν2 are respectively output to the (n−1)-th and n-th output ports and the beams ν3 to νn of light are respectively output to the first to (n−2)-th output ports.
A description will be given here of the operation of a known wavelength demultiplexer illustrated in FIG. 3 (PTL 1, NPL 1). The wavelength demultiplexer, which is constituted by an optical coupler (OC) and a 2×N (2-input N-output) AWG, performs the following operation. In this example configuration, for example, the AWG has 62.5-GHz-spacing inputs and 25-GHz-spacing outputs, and, at each output of the AWG, as illustrated in FIG. 3(b), a combination of a carrier and a sideband wave (for example, an upper sideband wave) is selected. This demultiplexer actively utilizes the narrow bandpass characteristics of the AWG.
Input signals in FIG. 3 are assumed to be wavelength-multiplexed signals arranged in the respective channels in an orderly way, as illustrated in FIG. 3(a). In some cases, however, as illustrated in FIG. 6, the signals may have (a) variations in the spacing between a subcarrier and a modulated wave or (b) signal strengths which differ from channel to channel. In such cases, in the illustrated wavelength demultiplexer, a subcarrier and a modulated wave having different signal strengths may be combined and the output of the wavelength demultiplexer is often output-channel-dependent.
The present invention is intended to achieve a stable separation output even for the wavelength-multiplexed signals illustrated in FIG. 6.